Aqueous coating binders based on epoxy resins have been used in the coatings industry for many years. Two-pack epoxy coating compositions, both solvent-borne and water-borne, are used particularly for heavy duty corrosion protection of metal substrates, with preference on base metals that do not have other corrosion protection such as zinc coating. Such systems dry quickly providing a tough protective coating with excellent hardness. Coating compositions based on epoxy resins are used mainly for factory-cast iron, cast steel, and cast aluminium parts. The use of water-borne epoxy resin systems reduces exposure and flammability issues associated with solvent-borne coatings, as well as liberation of solvents upon application. Coating compositions based on epoxy resins are mainly used in automotive and industrial applications, such as pipelines, and fittings, due to their heat resistance, chemical resistance and also, mechanical strength. Other uses of epoxy resin-based coating compositions are, for example, in can coating for acidic goods. Coating compositions based on epoxy resins are also widely used as primers to improve the adhesion of paints especially on metal surfaces in automotive and marine applications where corrosion resistance is important. They can also be used for high performance and decorative flooring applications such as industrial floorings, and architectural floorings such as terrazzo.
Water-based epoxy coating compositions usually comprise a hydrophilically modified epoxy resin, and a compatible curing agent which itself is also hydrophilically modified. This hydrophilic modification of epoxy resins is usually effected by introduction of non-ionic hydrophilic moieties. The reason is that the commonly used ionic hydrophilising groups such as amino groups or acid groups which form ions in aqueous environment are reactive themselves with epoxide groups. Introduction of the commonly used poly(oxyethylene) blocks as hydrophilising moiety is a difficult step as strong acid catalysts, mostly Lewis acids such as boron trifluoride, or complexes thereof with ethers or amines, have to be used, and the process is difficult to control. Such chemistry has been described in EP 0 272 595 B1, and also, in EP 0 346 742 B1, for epoxy resins, and i. a. in EP 0 000 605 B1, for curing agents based on adducts of epoxide-functional compounds and amines.
Amine based curing agents for epoxy resins usually have primary, secondary, or also tertiary, amino groups that react with an epoxide group under formation of a beta-hydroxy amine structure or a betaine structure. The curing activity decreases from primary to secondary to tertiary amines. While it is possible to use multifunctional primary amines which are the most efficient amines due to their higher reaction rate, such as isophorone diamine or meta-xylylene diamine, as curing agents for epoxide-functional compounds, their high vapour pressure and unfavourable smell together with potential health hazards has barred their use in applications where no sufficient ventilation is available. Moreover, lack of compatibility of monomeric amines with epoxy resins has limited their usefulness. Secondary amines which stem from reaction of primary amines with epoxide-functional compounds have good compatibility with epoxy resins, yet suffer from lower curing speed compared to that of primary amines.
It has therefore been the object of this invention to provide a multifunctional primary amine that can be used as curing agent in a curable epoxy resin system comprising an epoxide functional compound and an amine-based curing agent therefor, which has both good compatibility and fast curing speed in combination with the said epoxy resin, and further, dispenses with the need to hydrophilically modify both curing agent and epoxy resin.
This problem has been solved by providing hydrophilically modified multifunctional amines having at least two primary amino groups, which amines can be used as curing agents for epoxy resins. These amines can be combined with epoxy resins which need not have hydrophilic modification of their own, or at least no modification which suffices to disperse these epoxy resins themselves without the use of additional emulsifiers in an aqueous phase, and keep the epoxy resin stably dispersed in aqueous dispersions.
By a “hydrophilically modified” compound in the context of this invention, a chemical compound (including oligomeric or polymeric substances having a number average molar mass of at least 350 g/mol) is meant which comprises in its molecules, moieties of oligo- or poly-oxyethylene segments, optionally in mixture with polyoxypropylene segments, in a sufficient amount to keep the said compound stably dispersed in aqueous dispersion for at least one week at room temperature (23° C.), i. e. without phase separation or formation of precipitate visible with the naked eye.
Two substances X and Y are considered compatible for the purpose of this invention if they do not form separate phases after mixing and then letting rest for not less than one hour, in a mass ratio m(X):m(Y) of from 1:99 to 50:50, where m(i) is the mass of substance i, i standing for X or Y. A moiety Z is considered to be compatible with an other substance if a chemical compound consisting essentially of the moiety Z under consideration is compatible with the said other substance. A chemical compound W is considered to consist essentially of a moiety Z if the moiety Z under consideration provides a mass fraction of at least 50%, preferably of at least 70%, and more preferably, of at least 80%, of the said chemical compound W.